Current approaches to the assessment of molecular endpoints in disease usually require tissue and blood sampling, surgery, and in the case of experimental animals, sacrifice at different time points. Despite improvement in noninvasive imaging, more sensitive and specific imaging agents and methods are urgently needed. Imaging techniques capable of visualizing specific molecular targets and/or entire pathways would significantly enhance our ability to diagnose and assess treatment efficacy of therapeutic interventions for many different disease states. Unfortunately, most current imaging techniques report primarily on anatomical or physiological information (e.g., magnetic resonance imaging (MRI), computed tomography (CT), ultrasound). Newer modalities such as optical imaging, especially when combined with traditional imaging modalities such as MRI and new molecular imaging probes have the potential to revolutionize the way disease is detected, treated, and monitored.
Molecular imaging is a new field in the imaging sciences that transcends the traditional boundaries of imaging structure or physiology and has the potential to revolutionize current research and clinical practices towards real molecular medicine. The common paradigm for molecular imaging involves the use of a “molecular” probe or agent that selectively targets a particular gene, protein, receptor or a cellular function, with the absence, presence or level of the specific target being indicative of a particular disease state.
Specifically, optical imaging offers several strong performance attributes that make it a truly powerful molecular imaging approach, both in the research and clinical settings. Specifically, optical imaging is fast, safe, cost effective and highly sensitive. Scan times are on the order of seconds to minutes, there is no ionizing radiation, and the imaging systems are relatively simple to use. In addition, optical probes can be designed as dynamic molecular imaging agents that can alter their reporting profiles in vivo to provide molecular and functional information in real time. In order to achieve maximum penetration and sensitivity in vivo, the choice for most optical imaging in biological systems is within the red and near-infrared (NIR) spectral region (600-900 nm), although other wavelengths in the visible region can also be used. In the NIR wavelength range, absorption by physiologically abundant absorbers such as hemoglobin or water is minimized.
Many different types of optical imaging probes have been developed including (1) probes that become activated after target contact (e.g., binding or interaction) (Weissleder et al., Nature Biotech., 17:375-378, 1999; Bremer et al., Nature Med., 7:743-748, 2001; Meade, Conti et al., Nuclear Medicine and Biology, 30:261-265, 2003), (2) wavelength shifting beacons (Tyagi et al., Nat. Biotechnol., 18:1191-1196, 2000), (3) multicolor fluorescence probes (Tyagi et al., Nat. Biotechnol., 16:49-53, 1998), (4) probes that have high binding affinity to targets, i.e., that remain within a target region while non-specific probes are cleared from the body (Achilefu et al., Invest. Radiol., 35:479-485, 2000; Becker et al., Nature Biotech. 19:327-331, 2001; Bujai et al., J. Biomed. Opt. 6:122-133, 2001; Ballou et al. Biotechnol. Prog. 13:649-658, 1997; and Neri et al., Nature Biotech. 15:1271-1275, 1997), and (5) fluorescent semiconductor nanoparticles based probes (i.e., Quantum Dots Bawendi et al., J. Am. Chem. Soc., 125: 11466-11467, 2003).
MRI also has demonstrated potential for molecular imaging. A number of targeted MRI agents have also been developed for molecular imaging applications including (1) metal oxide magnetic nanoparticles (Gupta and Gupta, Biomaterials, 26:3995-4021, 2005; Weissleder, et al., Bioconjugate Chem., 13(1), 116-121, 2002; Weissleder, et al., Bioconjugate Chem., 13(2): 264-268, 2002; Josephson, et al., Bioconjugate Chem., 15(5):1062-1067, 2004; Josephson and Gaw, U.S. Patent Application Publication No. 2003/0124194), (2) emulsion “particles” consisting of a perfluorocarbon core surrounded by a lipid monolayer that are derivatized with targeting ligands and chelated gadolinium (Wickline et al., Circulation, 108: 2270-2274, 2003; Lanza, et al., Cancer Research, 63: 5838-5843, 2003); (3) probes that have high binding affinity to targets, i.e., that remain within a target region while non-specific probes are cleared from the body (McMurry et al., Curr Med Chem, 12(7): 751-78, 2005, and (4) probes that become activated after target contact (Curr Opin Neurobiol. 13(5): 597-602 2003).
In many research and clinical imaging settings it is desirable to have the ability to combine or make use of both high resolution anatomical and molecular information, provided by imaging techniques such as MRI, and molecular/functional information, provided by optical imaging approaches. Recent work in both clinical and pre-clinical imaging has integrated several modalities within a single system to great benefit compared with single-modality imaging systems and multi-modality imaging probes have been described (Meade et al., Bioconjugate Chem., 9(2): 242-249, 1998; Meade and Allen, Metal Ions Biol. Syst., 42: 1-38, 2004; Josephson et al., Bioconjugate Chem., 13(3); 554-560, 2002; Josephson et al., Bioconjugate Chem., 15(5); 1062-1067, 2004; Nie et al., U.S. Patent Appl. 2005/0130167; Hancu et al., WO2004026344).
To date however, the development of optical, MRI, and combined optical/MRI agents for molecular imaging applications has been limited by: 1) delivery barriers standing between molecularly targeted agents and their intended molecular target, 2) the fact that many monovalent ligands (antibodies, proteins, peptides) lack the affinity needed for efficient agent targeting, 3) limited target to background ratio due to sensitivity issues related to the signal strength of the individual agent (e.g., fluorescence or fluorescent brightness) of the individual agent, especially with regards deep tissue imaging (for optical agents) and low target abundance (MRI and optical agents), and 4) the limited biocompatibility and stability of some MRI (Palmacci and Josephson, U.S. Pat. No. 5,262,176) and optical agents (Bhatia et al., Nano Lett. 4: 11-18, 2004).
Thus, there is a need for biocompatible optical molecular imaging agents that can be specifically directed to a variety of molecular targets and provide high levels of sensitivity. In particular, there is a need for biocompatible multi-modality molecular imaging agents for in vivo imaging applications including in animals and in humans for disease detection, monitoring and assessing drug activities and therapeutic effects.